thats a good trick, as a matter of fact i always forget to try that in matches, anyways, you have to cancel into aegis on a late c.fierce (after first hit), this gives you time to dash past your opponent and create the unblockable.

My mid-screen unblockable…
cr.FP , EX Headbutt , cr.FPxxLP Aegis (I always just step forward and do the cr.FP so I’m unsure if it is technically the early or late hit but I know the only way to make it unblockable is to have them recover from the fall and land on their feet) then use the super at activation pause to pre-charge the forward should tackle you do just at the point of aegis appearing on screen . Then do toward+Forwrd kick for the quard break .